The laying of underground cables or pipelines is a multi-step process in which a trench is dug along the desired pathway of the pipeline. Typically the earth resulting from the digging of the trench is laid along one side parallel to the trench for latter use as backfill material. The underground cables and pipelines are emplaced by laying the cable or pipeline in the prepared trench and subsequently backfilling the trench.
Because some cables or pipelines are susceptible to damage by stones or other hard objects in the backfill material, a backfill or padding machine such as those described in U.S. Pat. Nos. 4,861,461; 4,912,862; 5,097,610 5,479,726; 5,540,003 are often utilized to separate the stones or other hard material from the relatively fine earth within the backfill material. This is typically done by way of a combination of screens and shakers in which the fine earth material is separated from the larger stones or other hard material. Such devices add to the cost of the pipeline laying process because of the additional equipment and manpower necessary to operate the machines.
Thus to maximize the life of a buried cable or pipeline it is desirable to backfill the trench with fill material that is relatively free of stone or other hard objects. One way to achieve this is to backfill the trench with sand or other suitable fill material brought from a remote source of sand or rock free soil. This approach is can prove to be very expensive, especially in remote areas, due to the time and costs involved in shipping the material to the work site. Further more where a steel pipe is covered with a layer of sand, the filled trench tends to accumulated water in the more porous sand filled trench which in turn leads to the premature corrosion of the pipe. Further the use of a fill material such as sand that is different from the surrounding soil may lead to a loss in the cathodic protection provided for the steel pipe, which also may lead to premature corrosion.
As previously noted, the alternative to is to screen the solid dug from the trench, remove the stones or other hard objects and return the remaining fine earth material to the trench. However, the cost of the additional equipment, time and manpower necessary to operate such equipment may equal or exceed the cost of bringing backfill material such as sand in from a remote source.
U.S. Pat. No. 5,398,430 issued to Scott et al. discloses a bucket and vibratory mechanism attachable to a backhoe. The bucket has a series of adjustable screening blades which alternatively can serve as either the bottom of the bucket or as a sifting mechanism for separating rocks and other hard objects from fine earth materials. While the use of vibratory mechanism to agitate the contents of the bucket is in theory functional, as a practical matter, the process of separation using the disclosed bucket is slow and inefficient.
U.S. Pat. No. 5,493,796 issued to Ballew et al. describes a pipeline padding system in which a backhoe operated bucket is modified so as to serve as a sifting mechanism for backfill material. The device described therein contains a vibrating screen device so that when the bucket is filled with backfill material, the screen is vibrated and fine earth backfill material falls into the pipeline trench. However, because the system utilizes a vibratory mechanism, the use of the described backhoe bucket is slow and inefficient.
U.S. Pat. No. 5,743,030 issued to Sirr discloses a front end loader with a dam shell type bucket which functions as a self cleaning particle size separation device. Although described as a clam shell bucket, the bucket described therein is actually a front end loader bucket which has been modified so as to sift backfill material that may be picked up by the bucket. As disclosed therein, a portion of the bucket is pivotably mounted so that material may be scooped up efficiently without premature sifting, the material is sifted, preferably by use of a vibrating mechanism, and then the bucket is cleared of larger debris. As with the other bucket based systems that utilize a vibratory mechanism, the process of separating padding material from stone material is slow and inefficient. Further because the bucket design is a front loader bucket, the tractor will at times need to drive or crawl into the backfill material which further compacts the material and makes the separation process more difficult.
U.S. Pat. No 5,771,612 issued to Lynch is generally directed to a loader bucket which has been modified so as to include a vibrational sifting system for separating rock from earth fine materials. The device disclosed therein includes a sifting devise that is pivotably mounted such that it can enclose a conventional loader bucket and by way of the vibrating means, sift the larger rock material from the finer earth materials. As with the other bucket based systems that utilize a vibratory mechanism, the process of separating padding material from stone material is slow and inefficient.
Despite the above devices, there remains a need in the pipeline and cable padding industry for new and improved methods and devices for sifting backfill material into rocks and other hard objects and fine earth materials used for padding the pipeline.